“Not sure about the UK but that is not the case in Canada.”
I think that it is difficult to closely compare systems in different countries since there are so few of us who went to school in multiple countries. I don’t think that I know anyone who did high school in both the US and in Canada.
I personally went to a relatively mediocre high school in Canada, and a couple of highly ranked universities in the US. I found that a mediocre high school in Canada did prepare me just fine for the US universities. Other than taking calculus my freshman year in university, while most of my classmates had AP’d out of it, I didn’t see anything academic that I wasn’t fully prepared for.
I see two big issues with the US system. One is that there seem to be less consistency in terms of the quality of various high schools across the country. The other is the enormous amount of stress that US suburban high school kids are put under. The stress seems to have several causes. However, to me one of the major causes is the scarcity of spots at the top ranked universities. A straight A high student in Canada doesn’t have to worry about which Canadian university they are going to get into, and also doesn’t have to worry about ECs. The top universities in Canada have enough spots for all straight A Canadian high school graduates (including some dual citizens moving up from the US). A straight A high school student in the US who wants to go to a highly ranked university in the US has difficulty predicting where they will get in, and has to worry about all kids of things other than just getting great grades and strong SAT scores.
Of course, things aren’t perfect in any country.